The present invention relates to a measurement of a specific component in a subject, for instance, blood sugar level of human or animal, by utilizing an attenuated total reflection (hereinafter referred to as "ATR").
Hitherto, various methods for detecting and determining a specific component in a living body, by using the ATR measurement apparatus have been proposed.
For instance, in Japanese Unexamined Patent Publication Hei 9-113439, there is proposed a method for measuring the blood sugar level by permitting a transparent ATR prism 50 having a pair of reflecting surfaces, which are parallelly confronting each other, to be in close contact with top and bottom lips as shown in FIG. 8. According to this method, while the ATR prism 50 is held in the mouth and pressed against the top and bottom lips, an incident beam is permitted to enter the ATR prism 50 and to repeat attenuated total reflections at the border between the reflecting surfaces of the ATR prism 50 and the lips, as designated by the broken line. The beam propagated through the ATR prism 50 is finally issued from the ATR prism 50. Information on specific substances in the lips can be obtained by analyzing the issued beam.
There is proposed another method in BME, Vol. 5, No. 8 (Institute of Japan Medical Electronics, 1991). This publication proposes a method for measuring the blood sugar level or blood alcohol level in which an ATR prism made of an optical crystal of ZnSe or the like is closely adhered to a mucus membrane of lips, then a laser beam with a wavelength of 9 to 11 .mu.m is permitted to enter the ATR prism and to have multiply internal total reflection inside the ATR prism. In this method, an absorbed light or the scattered and reflected light is analyzed. According to this method, a realtime measurement of the blood sugar level or blood alcohol level can be made in a non-invading manner.
These methods utilize an evanescent wave (so called "oozing light") for a quantitative analysis. That is, when a surface of the ATR prism 50 is pressed against a subject, a beam traveling through the ATR prism 50 totally reflects after slightly invading the lips at the surface as shown in FIG. 8. When the ATR prism 50 is sandwiched by lips, the beam invades the lips and is therefore affected by the respective components in a blood circulating through the lips. In this manner, by measuring an intensity of the reflected beam, it is possible to detect the change in the light transmittance, reflectance, absorbance, or the like of the blood, and thus the information concerning the respective components in the blood can be obtained.
However, the above-mentioned conventional ATR measurement apparatuses have the following problems.
It is hard to obtain the information on the deep spots of the living body, because the depth of the evanescent wave invasion is in an order of wavelength and the reflected beam contains a large amount of light which transmits through the spot near the surface. In particular, if any impurity exists on the border between the prism and the subject, the obtained signal is deteriorated.
Therefore, in the case of pressing the ATR prism against the lips as in the above-mentioned conventional examples, the adhesion between the surface of the ATR prism and the lips is not stable and thus it is difficult to conduct a measurement with a high accuracy. In addition, in a case where the saliva invades between the ATR prism and the lips, the measurement values are greatly affected by the saliva.
Further, an optical crystal of ZnSe, ZnS or the like which has widely been used as the conventional ATR prism for the above-mentioned measurement apparatuses has a very expensive price. In addition, it is difficult to apply such ATR prism to continuous measurements for a number of the subjects because the ATR prisms are very soft and require great care for their handling and washing.